Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Cash
Assets
Cash	$ 2,934	$ 1,519
Money market
Assets
Cash	83,257	23,994
Domestic corporate commercial paper
Assets
Cash	39,984
Government-sponsored enterprise debt securities
Assets
Cash	15,996
Domestic corporate debt securities
Assets
Cash	10,007	3,005
Asset-backed Securities
Assets
Cash	7,500
Recurring basis | Estimated Fair Value
Assets
Cash	159,678	28,518
Marketable securities	313,661	76,758
Recurring basis | Cash | Estimated Fair Value
Assets
Cash	2,934	1,519
Recurring basis | Money market | Estimated Fair Value
Assets
Cash	83,257	23,994
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Cash	39,984
Recurring basis | Government-sponsored enterprise debt securities | Estimated Fair Value
Assets
Cash	15,996
Recurring basis | Domestic corporate debt securities | Estimated Fair Value
Assets
Cash	10,007	3,005
Recurring basis | Asset-backed Securities | Estimated Fair Value
Assets
Cash	7,500
Recurring basis | Domestic corporate debt securities | Estimated Fair Value
Assets
Marketable securities		69,509
Recurring basis | Domestic corporate debt securities | Domestic corporate debt securities | Estimated Fair Value
Assets
Marketable securities	173,035
Recurring basis | Domestic corporate commercial paper | Estimated Fair Value
Assets
Marketable securities		7,249
Recurring basis | Domestic corporate commercial paper | Domestic corporate commercial paper | Estimated Fair Value
Assets
Marketable securities	84,158
Recurring basis | Asset-backed Securities | Domestic corporate commercial paper | Estimated Fair Value
Assets
Marketable securities	56,468
Recurring basis | Level 1
Assets
Cash	86,191	25,513
Recurring basis | Level 1 | Cash
Assets
Cash	2,934	1,519
Recurring basis | Level 1 | Money market
Assets
Cash	83,257	23,994
Recurring basis | Level 2
Assets
Cash	73,487	3,005
Marketable securities	313,661	76,758
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Cash	39,984
Recurring basis | Level 2 | Government-sponsored enterprise debt securities
Assets
Cash	15,996
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Cash	10,007	3,005
Recurring basis | Level 2 | Asset-backed Securities
Assets
Cash	7,500
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable securities		69,509
Recurring basis | Level 2 | Domestic corporate debt securities | Domestic corporate debt securities
Assets
Marketable securities	173,035
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable securities		$ 7,249
Recurring basis | Level 2 | Domestic corporate commercial paper | Domestic corporate commercial paper
Assets
Marketable securities	84,158
Recurring basis | Level 2 | Asset-backed Securities | Domestic corporate commercial paper
Assets
Marketable securities	$ 56,468